'Good Morning America's' Robin Roberts battling blood disorder

Robin Roberts, who overcame breast cancer five years ago, is battling another health scare. The “Good Morning America” anchor announced Monday that she has myelodysplastic syndrome, a disease of the blood and bone marrow, that was likely caused by her cancer treatment.

The 51-year-old anchor learned of the diagnosis on what was supposed to be a victorious day: when "GMA" beat NBC’s “Today” show in ratings for the first time in 16 years, she said in a statement on the network’s website: "Talk about your highs and lows!" Roberts said. She expects to undergo a bone marrow transplant this fall, with her sister as her donor.

"My doctors tell me I'm going to beat this — and I know it's true," she said in the statement. “ If you Google MDS, you may find some scary stuff, including statistics that my doctors insist don't apply to me. They say I'm younger and fitter than most people who confront this disease and will be cured."

Roberts will begin chemotherapy immediately. The network's medical correspondent, Dr. Richard Besser, also said in a statement on the website that he was consulting with Roberts about MDS, a malignant disorder of the bone marrow, sometimes known as pre-leukemia, that typically affects older people and can also be triggered by cancer treatment.

Roberts will continue appearing on the morning show but is expected take several months off after the transplant to recover.